Problem with solving LN equation

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How to solve that equation and write answer on field C7? If i change other fields i want to C7 changes, it gives me DIV/0 error, solver also gives me 0, but the answer is 8.8 or 180

not every1 can open .png files, myself included. It would be far better if you could upload a sample of your file, showing what you have, and an example of your expected outcome. Thanks
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looking at you're file (and the formula above), unless you enter a value in C7, you will always get a div/0 error, because thats exactly what's happening....

.......LN(C4/C7).........
.......LN(C4/0).........

I think your formula has an error (meaning it's not the correct formula).

What is the problem you're trying to solve -- something about acoustic resonance?

A couple of thoughts
1) You have to put a value of some kind in for d. Solve defaults to the newton raphson method, which requires an initial guess for d. A blank cell is assumed to be 0, which gives an error, and Solver can't do anything with an error value.
2) If I manually put 8.8 in for d, I don't get anything near 0. Further testing suggests that there are no (real) zeros for the equation as entered. I'm beginning to suspect an error in the formula.
3) If I algebraically solve the equation in the latest picture for d (+ and - sqrts), I get 8.9 and 180 for the roots. further evidence that the entered formula contains an error. Whether solved numerically or algebraically, we should get the same roots
4) A careful look at the formula indicates that you have misentered the parentheses. you have c/2*f (=c*f/2) when the equation calls for c/2/f (=c/(2*f)). No wonder you are having trouble, you were multiplying when you should have been dividing. Entering complex formulas (with lots of parentheses and lots of terms/operations) is always going to be a challenge, and I don't know that there is any magical solution for the problem. We just have to practice and develop skills for entering these kind of formulas accurately, because Excel is just not smart enough to tell when we've made this kind of mistake in a formula.

With some algebra, you can solve this in closed form:

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